1. Field of the Invention
The present invention is concerned with novel chiral dopants for liquid crystals as well as liquid crystalline mixtures which contain such dopants and their use for optical and electro-optical purposes.
2. General Discussion
Liquid crystal materials for electro-optical indicating devices frequently contain one or more optically active additives for the induction of a chiral structure. For example, for use in indicating devices having a twisted nematic structure a nematic liquid crystal is doped with an optically active additive, for example in order to prevent a reversal of the twisting direction (reverse twist) in TN cells (twisted-nematic) or in order to achieve a sufficient twisting in cells having a highly twisted nematic structure such as STN cells (super twisted-nematic), SBE cells (super birefringence effect) or OMI cells (optical mode interference). Further, cholesteric liquid crystals for phase-change cells can preferably consist of a nematic basic material and one or more optically active dopants and ferroelectric liquid crystals for indicating devices based on chiral tilted smectic phases can preferably consist of a material having a twisted smectic phase and one or more active dopants.
The electro-optical characteristics of liquid crystal indicating devices are temperature-dependent, which is especially troublesome in the case of multiplex operation. It is, however, known that this temperature dependence can be compensated for at least partially by the addition of chiral dopants which induce a decreasing pitch with increasing temperature. Such an inverse temperature dependence has been found only for a few compounds. It can, however, also be achieved by using at least two chiral dopants which have a different relative temperature dependence and which induce a different twisting direction (U.S. Pat. No. 4,264,148). Of course, this requires for the most part a relatively high amount of chiral dopants.
Cholesteric liquid crystals reflect light in a wavelength range for which the wavelength is approximately equal to the helical pitch. The spectral width of this reflected light can be varied by a suitable choice of the liquid crystal. The reflected light is completely circularly polarized. The direction of rotation of the reflected light depends on the direction of rotation of the cholesteric helical structure. The light circularly polarized in the opposite direction is transmitted without adsorption. These properties can be utilized for the production of optical filters, polarizers, analyzers etc. Further, cholesteric liquid crystals have also variously been used for thermochromic applications and in cosmetic preparations.
Cholesteric liquid crystals for the above applications can o preferably consist of a nematic or cholesteric basic material and one or more chiral dopants, which permits a simple adjustment of the desired helical pitch.
In order to produce cholesteric mixtures having a pitch in the range of the wavelength of visible light, the chiral dopants should have a twisting capacity which is a high as possible and should have a good solubility in usual liquid crystal materials. Furthermore, the chiral dopants should have an adequate stability, should have a good compatibility with the mesophase type of the liquid crystal material and should not restrict the mesophase range too stongly. Such properties would also be desirable for chiral dopants for producing the twisted nematic structures referred to earlier, since their amount can be held low in order that the properties of the liquid crystal material are influenced only immaterially. The use of chiral dopants in ferroelectric liquid crystals serves primarily to produce to change a twisting of the tilted smectic phase and to induce a spontaneous polarization. The chiral dopants should have an adequate stability, should have a good compatibility with the tilted smectic liquid crystal and should not restrict the mesophase range too strongly. Further, it would be desirable for the twisting and the spontaneous polarization to be adjusted independently of each other as far as possible and for the viscosity of the chiral dopants to be comparatively low.